1. Field of the Invention
The present invention relates to a fluid coupling and, more particularly, to a fluid coupling having no inner core and suited for use with a continuously variable transmission.
2. Description of the Prior Art
A fluid coupling (hereinafter referred to as "coupling") operates to transfer power through a working fluid between a pump impeller and a turbine runner, which are opposed to each other. The coupling does not function to increase input torque, and is different from a torque converter in this regard; rather, a coupling simply functions as a power transmitting junction. Since a coupling can be made smaller and lighter that a torque converter, due to absence of a stator, it is well suited for use as a coupler in a vehicle having a V-belt type continuously variable transmission (referred to hereinafter as "CVT").
Couplings of two types are known. One type of coupling is equipped with an inner core for guiding the flow of a working fluid therein and the other type is not equipped with such an inner core. The coupling of the type having no inner core offers the advantage of low weight, although its fluid passage varies in accordance with slippage between the pump and the turbine. In not having a fixed flow passage, it differs from the type of coupling having an inner core, and flow analysis for such a coupling is difficult, as is designing for predictable performance. An "inner core" is shown, for example, as member 8 in U.S. Pat. No. 5,005,356 issued to Saunders and as elements 10b in U.S. Pat. No. 4,866,935 issued to Hayabuchi et. al.
If a coupling without an inner core is to be used in an automobile to exploit its advantages, in order to improve the starting acceleration of a vehicle having a torque converter, especially a CVT, the engine r.p.m. at the start is desirably raised to increase torque, by reducing the capacity coefficient of the coupling at stalling (at a velocity ratio of 0) time. On the other hand, the capacity coefficient in an intermediate velocity ratio range has to be high to provide good mileage in steady running, good passing acceleration and good responsiveness.
The conventional approach to adjusting the capacity coefficient in a coupling having no inner core is by changing the blade angle. If the capacity coefficient at stalling is decreased in such a manner, the capacity coefficient in the intermediate velocity ratio range decreases with an increase in the velocity ratio, but the maximum capacity coefficient range is not in the intermediate velocity ratio range. In short, the characteristics cannot be finely adjusted by changing the blade angle although the capacity coefficient can be changed.
It is known in the prior art to provide a baffle plate for setting the capacity coefficient to a low value at stalling, while suppressing reduction in the capacity coefficient in the intermediate velocity ratio range. With a baffle plate, the flow of the working fluid in the coupling is blocked over a range extending from the time of stalling to the low velocity ratio and is offset from the blocked path in an intermediate or higher range so that the blocking action may be reduced or eliminated.
However, in attaching a baffle plate to the blade, the number of parts, the weight and the production cost are increased accordingly. This prior art approach also creates a problem in that the baffle board is difficult to fix, thereby endangering the reliability of the structure.